Add read file in bytes using fileb://

Fixes https://github.com/aws/aws-cli/issues/814, https://github.com/aws/aws-cli/issues/815

If you want to read the file as binary, simply prepend file name with ``fileb://`` instead of ``file://``

cc @jamesls @danielgtaylor 
Yeah I'd test to make sure, but I think just using %r always would be easier to maintain.  It's a debug log anyways so we're free to alter the messages and the formatting.